In an electrophotographic image forming apparatus, light emitted from a light source based on image data is scanned over a photoconductor drum by a rotary polygon mirror, and an electrostatic latent image corresponding to the image data is formed on the photoconductor drum. In this type of image forming apparatus, a light emission timing corresponding to each line of image data, namely, an electrostatic latent image writing timing is determined based on a light detection timing at which the light emitted from the light source and scanned by the rotary polygon mirror is detected by a light detecting portion.
In addition, there is known a related technique configured to identify a reflection surface of the rotary polygon mirror based on the light detection timing at which the light is detected by the light detecting portion, and an output timing at which a drive signal is output, the drive signal being to be input to a drive motor that rotates the rotary polygon mirror (see PTL 1). Specifically, according to the related technique, an interval between a detection timing at which light is detected by the light detecting portion and an output timing at which a drive signal is output, is measured in each detection cycle in which light is detected by the light detecting portion, and a reflection surface of the rotary polygon mirror is identified based on whether a measured interval matches a reference interval that corresponds to a predetermined reference reflection surface.